The present invention relates generally to metal presses used to form sheet metal into various shapes, and in particular to an improvement in a press used to form a hem along a longitudinal edge of a piece of sheet metal.
A hem is a seam wherein the edge of a piece of material is folded back onto the body of the material to form a stronger edge along the piece of material. In sheet metal fabricating, a hem is formed when a longitudinal edge of a piece of sheet metal is bent back toward the body of the sheet metal and flattened down to form a flat border along the edge of the sheet. This flat border, or hem, has a thickness which is slightly greater than twice the thickness of the sheet metal.
In the past, a hem has been formed along the edge of a piece of sheet metal by a two step process. A primary bending die has been provided along the lower longitudinal edge of the ram of a metal press, or press brake. This primary die is roughly "U-shaped" in cross section. A matching bottom die has been provided on the working surface of the work table. This bottom die is roughly "V-shaped" in cross section and is adapted to receive the body of the primary die within the interior of the "V". The coaction of the primary and bottom dies form what is known as a "full bend" along an end of the sheet metal. A "full bend" is a roughly hook-shaped bend formed along the edge of the sheet metal, the actual edge of the sheet comprising the end of the hook which is directed back over the body of the sheet.
Initially, an individual work piece was placed on a working surface of the work table of a metal press. The metal press, or press brake, was then engaged so that the primary die of the large, roughly rectangular ram was lowered into contact with the bottom die provided along the work table. When the ram of the press brake was lowered so that the primary die engaged the bottom U-shaped die, the sheet metal placed on top of the bottom die was deformed into a "full bend".
In the past, if it was necessary to form a hem on a large number of pieces of sheet metal, a full bend was first formed on each individual piece. Then, after a full bend had been formed on each individual piece a beam-shaped fixture was manually lifted into place, and bolted onto the face of the ram of the press brake. The beam-shaped member was positioned so that the bottom most portion of the beam coacted with the work table of the press brake. Each individual piece of sheet metal was again inserted into the press brake so that the longitudinal edge of the sheet, having the full bend formed on it, was flattened between the bottom of the beam-shaped fixture and the work table as the ram was again lowered.
The above described method of forming a hem is cumbersome and time consuming. Further, the above method is labor intensive in that it requires a number of workers to lift the beam shaped fixture into place and hold it there while it is bolted to the face of the ram. A similar number of workers are required to remove the beam-shaped fixture from the ram. Finally, the above method of forming a hem has proven to be inefficient if the pieces of sheet metal must undergo further fabrication or treatment after the hem has been formed. Production on the individual pieces is held up until the entire lot of sheet metal undergoes both the full bend and hemming procedures.
Thus, the prior art methods and devices used to form a hem on a piece of sheet metal have demonstrated various disadvantages which have heretofore not been overcome.